The present invention relates generally to cellular, personal communication services (PCS) and other wireless communication systems, and more particularly to techniques for estimating the location of a mobile telephone or other mobile communication station in such systems.
Demand for wireless communication services, such as mobile telephone services in cellular and PCS systems, is continually increasing. An important issue in wireless communication systems involves the estimation of mobile station location. For example, the Federal Communications Commission (FCC) has requested that all cellular and PCS systems eventually include emergency 911 caller location capabilities similar to those provided in wired systems. As noted in Radio Communications Report, Vol. 15, No. 51, Dec. 16, 1996, the FCC has required that Phase I of a wireless emergency 911 (E-911) system providing a 911 agent with caller number and cell site location must be completed by Apr. 1, 1998, while Phase II of the E-911 system providing caller latitude and longitude within a radius of no more than 125 meters in at least 67% of all cases must be completed by Oct. 1, 2001. A number of other services requiring mobile location estimation are also being considered, including routing guidance services, fleet management and other commercial services. A wireless system which is able to determine the position of a given mobile station in an efficient manner could thus provide an enhanced level of service to the user, while meeting the above-noted FCC requirements and also generating additional revenue for the service provider.
Many conventional techniques for estimating mobile location in a wireless system are based on time difference of arrival (TDOA) measurements, which involves measuring the difference in arrival time of signals transmitted to or from different locations in the system. When implemented in the base-to-mobile direction, TDOA location estimation involves the mobile station detecting signals transmitted from at least three surrounding base stations. When implemented in the mobile-to-base direction, TDOA location estimation involves at least three surrounding base stations detecting a location signal transmitted from the mobile station. In either case, the resulting signal arrival time information can then be processed using well-known relationships to derive an estimate of mobile location. Three base stations are generally required in order to estimate mobile location in two dimensions. Differential range values may be computed by multiplying the TDOA differential path delay measurements by the speed of light c to provide an estimate of the differential distance between the mobile station and any pair of the three base stations. Each differential range defines a hyperbola having its foci at the corresponding base stations, such that the mobile location may be estimated as the intersection of three hyperbolas associated with the three pairs of base stations. A two-dimensional mobile station location estimate (x0, y0) may be generated by solving equations for the hyperbolas using differential range values computed for the first and second, first and third and second and third base stations. Additional details regarding these and other conventional location estimation techniques may be found in, for example, J. Caffery et al., xe2x80x9cRadio Location in Urban CDMA Microcells,xe2x80x9d Proceedings of PIMRC ""95, pp. 858-862, IEEE, 1995, and M. Wylie et al., xe2x80x9cThe Non-Line of Sight Problem in Mobile Location Estimation,xe2x80x9d ICUPC ""95, Boston, Mass., 1995, both of which are incorporated by reference herein.
Regardless of the manner in which the differential range values are processed to determine mobile location, the TDOA location estimation process is often complicated by the limited bandwidth available for the time difference measurements. In general, a wider bandwidth provides a more accurate arrival time measurement for a given location signal. The multipath environment in which many wireless systems operate further complicates the measurements. The arrival time of interest for a given location signal is that corresponding to the most direct path between a transmitter and receiver. This arrival time will be delayed by an amount of time proportional to the distance or range between the transmitter and receiver, and is therefore useful in location estimation. The signals carried by other non-direct paths can be regarded as a form of noise. Another source of impairment is additive noise due to interference from other mobile stations or base stations. For example, when a mobile station is within about one-fourth of a cell radius of a given base station, a location signal transmitted from or received by that base station can be as much as 35 dB stronger an the corresponding signal associated with the third nearest base station. Moreover, in wireless systems based on the IS-95 standard, the weaker signals will generally occupy the same frequency band as the stronger signals. In the same band there will also be signals transmitted to or received from other mobile stations, further degrading the signal-to-interference ratio (SIR). Under these conditions, successful reception of the weaker location signals becomes increasingly difficult as the mobile gets closer to a given base station.
It is therefore important that a system with TDOA-based mobile location estimation provide sufficient bandwidth to discriminate a desired direct path location signal from the unwanted multipath signals, and sufficient signal-to-noise ratio (SNR) to enable the possibly very weak direct path signal to be detected in the presence of interference from other mobiles or base stations. Unfortunately, calculations indicate that the 1.25 MHz bandwidth typical of a given IS-95 communication channel is not adequate to achieve the desired TDOA accuracy. For example, a bandwidth of about 10 MHz would typically be required to provide a 100 ns resolution for the TDOA measurements. In addition, the fact that many IS-95 system base stations utilize the same frequency bands also makes the available SNR inadequate. Application of conventional TDOA-based mobile location estimation techniques to IS-95 and other CDMA wireless systems may therefore require alteration of basic system parameters, thereby increasing the cost and complexity of the system and possibly degrading system performance in terms of interference and voice quality.
As is apparent from the above, a need exists for a mobile location estimation technique which provides a broader bandwidth and enhanced SNR for transmitted location signals, without altering basic system operating parameters, without requiring substantial additional mobile station and base station circuitry, and without significantly degrading voice quality and other performance measures in the wireless system.
The present invention provides methods and apparatus for mobile location estimation in a wireless communication system. An exemplary base-to-mobile embodiment of the invention involves the transmission of location signals during periodic system-wide blank-and-burst intervals in which voice data communications and other on-going wireless communications are temporarily suspended. For example, a subset of the base stations of the wireless system may each transmit a distinct location signal during the blank-and-burst interval. The blank-and-burst interval may be selected to have a duration of about 1 to 100 milliseconds such that voice quality and other system performance measures are not significantly degraded. A particular mobile station for which a location estimate is to be generated may then detect the three distinct location signals from nearby base stations, and generate TDOA differential path delay measurements which are used in triangulation or another suitable technique to generate the location estimate.
The location signals transmitted during the blank-and-burst interval in this exemplary base-to-mobile embodiment may occupy the bandwidth of multiple channels of the system, and will thus have a broader bandwidth than would otherwise be possible using conventional techniques. The location signals may be configured using predetermined waveforms which carry no information other than that provided by the waveform itself. In addition, the location signals may be selected so as to appear mutually orthogonal at a receiving mobile station, such that the SNR of the location signals during the blank-and-burst interval is substantially increased. One possible technique for ensuring mutually orthogonal location signals involves selecting the signal waveforms such that the signals exhibit substantially non-overlapping comb-like frequency spectra. Other techniques may also be used to generate mutually orthogonal location signals in accordance with the invention. For example, in an IS-95 or other similar system in which base stations transmit a pilot signal for synchronization purposes, the pilot signals may be used as location signals. The orthogonality feature is provided in such an embodiment if the mobile station receiver integrates the received signal over a time interval that is a multiple of the repetition period of the pilot signals. Some or all of the base stations could therefore suspend all communications during the blank-and-burst interval except for transmission of the pilot signal.
An alternative implementation of the above-described base-to-mobile embodiment may blank only a portion of the total bandwidth utilized by the wireless system. Such an implementation is particularly well-suited for use in channelized wireless systems such as the Advanced Mobile Phone System (AMPS) and IS-136. For example, an embodiment may be configured in which only every fourth channel can be used for mobile location estimation, in order to preserve three-fourths of the system bandwidth for uses unrelated to location estimation during any given blank-and-burst interval. This partial blank-and-burst technique may be implemented in a CDMA system by applying a suitable notch filter to an otherwise normal CDMA signal prior to its transmission during the blank-and-burst interval. The notch filter will generally not affect portions of the CDMA signal used for other purposes, but frees up the portion of the bandwidth needed to transmit the base-to-mobile location signals.
The invention may also be implemented in a mobile-to-base embodiment in which at least a subset of all system mobile stations are directed to terminate ordinary voice data communication functions for a short blank-and-burst interval on the order of 1 to 100 milliseconds. A system controller then directs one or more mobile stations to transmit a location signal during the interval. The location signal has an identifying characteristic, such as a particular one of a number of different possible comb-like frequency spectra, which associates it with its corresponding mobile station. The location signal is received by at least three base stations in the vicinity of the transmitting mobile station, and the various detected versions of the transmitted location signal are processed to generate an estimate of mobile location. As in the base-to-mobile embodiments, the location signals in this mobile-to-base embodiment may be configured to occupy a broad bandwidth, such as the bandwidth normally occupied by multiple channels of the wireless system. The system controller may assign particular types of location signals to be used by particular mobile stations for a given blank-and-burst interval so as to preserve the above-described appearance of mutual orthogonality for that interval. The location signal assignments may differ from interval to interval depending on which mobiles will be using the location estimation feature. At the expiration of the system-wide blank-and-burst interval, the system mobile stations resume normal voice data communications.
The present invention provides mobile location estimation in a manner which permits location signals to be transmitted with a substantially broader bandwidth and increased SNR than conventional techniques. Moreover, the improvements are provided without the need for significant alteration in basic system operating parameters or large amounts of additional base station or mobile station circuitry, and without significantly degrading voice quality or other system performance measures. These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.